The present invention relates to a method of precision measurements of sizes and line width roughness of small objects in accordance with their images obtained in scanning electron microscope.
Since size of any object is a distance between its edges, therefore in accordance with an accepted statement a problem of localization of edges of the measuring object is a main problem of a size metrology as a whole. As stated in M. Postek “Critical Issues in Scanning Electron Microscope Metrology”//Journ. of Research of the Nat. Inst. of Standards and Technol., Vol. 99, No. 5, 1994, p. 656. “without an ability to know with certainty the location of the edges, measurement accuracy can not be claimed”
During measurement of objects of small sizes, such as elements of integrated circuits, scanning electron microscopes are widely utilized. In this case it is necessary to solve the problem of localization of an edge of an object on its SEM image. This problem in the present time is not solved.
The existence of mathematical models of processes for forming a video signal in SEM opens new possibilities of localization of an edge and, correspondingly, of measurements of microsizes. Several models are disclosed in corresponding literature, which allow imitation of the SEM video signal with a varying accuracy. The accepted approach to the use of mathematical models in metrology can be explained as follows:
Let is assume that there is a model which allows to imitate the video signal in SEM accurately. Then, it is possible to replace the experimental video signal Se(x) obtained in SEM from the measuring object, with a model video signal Sm(x) without losses or distorted information contained in Se(x). The meaning of this replacement is that on the signal Se(x) a position of the edge of the object is not known, while on Sm(x) it is known, since this value was given as a parameter during calculations of the video signal Sm(x) coinciding Se(x).
Models which are widely used are those that are based on a calculation of trajectories of the separate electrons in a solid body in accordance with the method of Monte Carlo as disclosed in J. Lowney, M. Postek, and A. Vladar. “A Monte-Carlo model for SEM linewidth metrology”.//SPIE Proc., 2196, 1994, pp. 85-96. In J. Villarubia, A. Vladar, J. Lowney, and M. Postek. “Scanning electron microscope analog of scatterometry”//Proc. of SPIE, Vol. 4689, 2002, p. 306. Approaches described in accordance with the opinion of their authors are capable of solving the problems of measurements of microsizes. These approaches are however not free from disadvantages and limitations. Some of them are as follows:
Achievement of acceptable statistic accuracy of the results requires great calculating resources, and the time spent for calculations is significant. A typical time for the calculation of the shape of one edge is several hours. Therefore a possibility of wide varying of the parameters of a model is limited. The authors see the solution that, preliminarily, a “library” of variants is to be calculated from which when necessary it is possible to select needed variants for comparison then with the experimental video signals. If the number of the parameters of the model is N, then for each of them, for example for a size, 10 different values are calculated and the total number of library variants is 10N. The number of parameters of the model can reach 7-8, so that the total number of “cards” in a card system of such a library can be tens of millions. The library with such a capacity is very difficult to create and it is more difficult to use it. The dividing of the range of sizes, for example into two parts, can not provide an accuracy of determination of the sizes which are necessary for the modern technology. A more detailed division would lead to a significant increase of the value of the library which is already very high.
There is another approach to modeling of the video signal in SEM, which don't use the method of Monte Carlo. The video signal in this case is calculated based on analytical (algebraic) expressions which are based on tested laws of interaction of electrons with a solid body, generation of secondary electrons, and their absorption in a thickness of the object and exiting through a shaped surface as disclosed for example in R. Ammossov, V. Dgeleznov, A. Nikitin, “Forming of the SEM image from trench with vertical sidewalls”.//Electronnaja Technika, ser. Mikroelektronika, vyp. 3(115), 1985, pp. 16-23. (In Russian), R. Ammossov, V. Dgeleznov, A. Nikitin. “Analys of the SEM-images of a ledge with inclined sidewalls”.//Electronnaja Technika, ser. Mikroelektronika, vyp. 3(119), 1986, pp. 17-21. (In Russian). For calculation of a model video signal in these cases it is not necessary to carry out excessive calculation resources, and the time for calculation of a signal variant corresponds to milliseconds. Therefore it is possible to get rid of libraries and to carry out a comparison of experimental and model video signals directly during the process of performing measurements, with execution of several clarifying interpolations. As a result, it is possible to achieve a high accuracy of measurement. Adequacies of the model signals to experimental signals can be evaluated by a comparison of FIGS. 1a and 1b. 